1. Field of the Invention
The invention relates to improved membrane electrode assemblies for solid polymer electrolyte fuel cells and, in particular, to electrode formulations for catalyst coated membrane assemblies that result in improved performance characteristics.
2. Description of the Related Art
Fuel cell systems are presently being developed for use as power supplies in a wide variety of applications, such as stationary power plants and portable power units. Such systems offer the promise of economically delivering power while providing environmental benefits.
Fuel cells convert fuel and oxidant reactants to generate electric power and reaction products. They generally employ an electrolyte disposed between cathode and anode electrodes. A catalyst typically induces the desired electrochemical reactions at the electrodes. A preferred fuel cell type, particularly for portable and motive applications, is the solid polymer electrolyte (SPE) fuel cell which comprises a solid polymer electrolyte membrane and operates at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the cathode and anode. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte membrane. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain an ionomer similar to that used for the solid polymer electrolyte membrane (e.g., Nafion®). Porous, electrically conductive substrates are typically employed adjacent to the electrodes for purposes of mechanical support, electrical conduction, and/or reactant distribution. These substrates thus serve as fluid diffusion layers.
MEAs can be fabricated by first applying a catalyst layer to a porous, electrically conductive substrate to form a fluid diffusion electrode. The fluid diffusion layer is then bonded to the membrane electrolyte. Alternatively, the catalyst layer may be applied directly to the membrane electrolyte instead to form what is known as a catalyst coated membrane. Either approach may be used for either or both electrodes in making an MEA.
An SPE fuel cell also typically employs flow field plates for directing the reactants across one surface of each electrode or electrode substrate. The flow field plates are disposed on each side of the MEA. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell series stack.
During normal operation of an SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water reacton product.
While significant advances have been made in this field, there remains a need for improved electrode assemblies.